1. Field of the Invention
This invention relates generally to electric motors, and more particularly to a rotor for electric motors in which rotor magnets and a rotor frame are integrated by a resin and a method of making the rotor.
2. Description of the Prior Art
The prior art has provided outer rotor type electric motors in which a bottomed short cylindrical rotor having a relatively large diameter is used. The rotor comprises a bottomed short cylindrical frame having a large diameter and made of a steel plate and a number of rotor magnets mounted on an inner circumferential surface of an annular wall of the frame. With regard to mounting the rotor magnets on the inner circumferential surface of the frame wall, the prior art has proposed a method of integrating the rotor magnets and frame by means of a resin. More specifically, the rotor magnets and frame are accommodated in a cavity of a forming mold. Subsequently, a molten resin is poured into the cavity to be hardened so that the rotor magnets and frame are enclosed in the hardened resin to be integrated together.
The thickness of the annular wall of the frame needs to be increased in the above-described rotor so that magnetic paths are sufficiently secured for the rotor magnets. However, the increase in the thickness of the annular wall results in an increase in the thickness of a bottom wall of the frame. That is, the bottomed short cylindrical frame is formed by pressing a steel plate and accordingly, the thickness of the steel plate needs to be increased so that the thickness of the annular wall of the frame is increased. This also increases the thickness of the bottom wall of the frame. Thus, the entire thickness of the frame is increased. This disadvantageously increases the entire weight of the rotor.